The Centers for Disease Control and Prevention on Friday released new details about cases of sexual transmission of the Zika virus in the United States, as well as information about nine pregnant women who contracted Zika while traveling.

Of the 14 cases of suspected sexually-transmitted Zika the CDC said it was investigating earlier this week, two have been confirmed and another four are probable, suggesting that infected men can pass the virus on to sexual partners at higher rates than previously thought.

Several of the sexual partners infected were pregnant women, although the CDC did not specify how many.

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The CDC is still investigating six of the cases, and has ruled that two of the instances were not cases of sexually-transmitted Zika.

CDC director Dr. Thomas Frieden told reporters on Friday that officials had anticipated Zika would pop up among US residents who traveled to Zika-affected areas. “We did not, however, anticipate that we would see this many sexually-transmitted cases of Zika,” Frieden said.

A majority of people who contract Zika do not show symptoms, which include fever, rash, conjunctivitis, and body and head aches. But all of the documented cases of sexually-transmitted Zika to date came from men who were showing symptoms or who had just recovered from their symptoms, the CDC said. Studies are underway to determine how long the virus can persist in semen.

Men who travel or live in areas where Zika is being spread should abstain from sex or use condoms during vaginal, anal, and oral sex with pregnant women, the CDC has recommended.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions.Alex Hogan/STAT

A woman who contracts Zika during pregnancy is suspected to be at higher risk of having a baby born with a condition called microcephaly, in which the child has an underdeveloped head and brain, and possibly other birth defects. A case study published this week found that a pregnant woman in Brazil infected with Zika had a stillborn baby with severe loss of brain tissue and abnormal buildups of fluid.

Scientists are still trying to determine if Zika can definitively cause microcephaly, although, Frieden said, “the evidence for this is getting stronger by the day.”

Also Friday, state officials in Oregon announced a case of sexually-transmitted Zika had occurred there, although CDC officials would not confirm if that was one of the cases they described in their new reports.

Zika in pregnancy

The CDC has also confirmed Zika infections in nine pregnant US residents who traveled to places with Zika transmission. Officials are investigating another 10 pregnant women for infection.

Among the nine women, two are still pregnant with no complications to date, two have miscarried, and two aborted their pregnancies; two others gave birth to healthy babies and one birthed a baby with severe microcephaly.

The woman who had a baby with severe microcephaly was likely infected with Zika while living in Brazil for the first 12 weeks of her pregnancy. (It is thought that infections that occur during the first or early second trimester are more likely to cause birth defects.) As a newborn, the baby suffered from seizures, abnormally tight muscles, and trouble swallowing.

In another case reported by the CDC, a woman with a confirmed Zika infection had an abortion at about 20 weeks after an ultrasound showed the fetus lacking parts of its brain; she had traveled to a Zika-affected place between 11 and 12 weeks of pregnancy.

In the two instances of miscarriage, scientists identified Zika virus RNA in the fetal tissue or placenta, but the CDC said it was not known if Zika caused the miscarriages, which are somewhat common during the first trimester.

The CDC has large studies underway in Colombia and Brazil to determine what percentage of pregnant women infected with Zika will have malformed fetuses, Frieden said. The nine cases described Friday are not enough to draw larger conclusions.

The CDC has recommended that pregnant women avoid traveling to Zika-affected places if possible. For those who must or those who live in such places, the CDC says they should do their best to avoid mosquito bites.

Zika and Guillain-Barré syndrome

Public health officials are also investigating whether the Zika virus can cause Guillain-Barré syndrome, which triggers a paralysis that is generally temporary but that can lead to death. On Friday, the World Health Organization announced that rates of GBS, which are usually around 1 to 2 for every 100,000 people, had spiked in six Latin American countries affected by the Zika outbreak.

As many as two-thirds of the Guillain-Barré patients in South America had symptoms consistent with a Zika infection in the few weeks prior to coming down with the syndrome, Dr. Tarun Dua, WHO’s expert on neurological disorders, said in a Friday news conference.

But she and other health officials said it is still too early to conclude that Zika causes Guillain-Barré. “Work is ongoing to look at the causality,” she said.

There have also been anecdotal reports of other neurological problems possibly associated with Zika, she said, including brain swelling and issues with nerves in the spinal cord and eyes.

“There could be other neurological complications which we don’t know,” she said, adding that researchers are looking at that now. “We need to understand the scope of consequences that might be happening in the context of this public health emergency.”